The 13ft barrier, dubbed The Great Wall of Calais, was built using £2.3million of UK taxpayers’ money.
It runs for well over half a mile beside the N216 motorway, which ends at the town’s ferry port terminal.
But a probe has found that steel doors set into it as access points for French officials are a glaring weak point.
On the side of the wall facing away from the road the doors look like featureless sheets of plain metal.
But on the traffic side they have handles with simple catches which can be opened without a key.
Migrants have discovered that all they have to do is gather at a door and send one of their mates running to the end of the wall.
He skirts round it, hares back to the door and opens it to let them all pour through to the road where they can jump on trucks – the very thing the wall was designed to prevent.
The embarrassing security lapse is said to be an open secret among Calais’ community of migrants desperately trying to get to Britain illegally.
An investigative team visited the area and saw first-hand how the doors can be opened in as little as 20 seconds.
A migrant in a camp in woodland near where the wall ends, at a roundabout next to Rue Yervant Toumaniantz, said it was “easy” to beat security.
He boasted: “They can’t stop us getting on to the lorries. We will always find a way to get to England.”
A source said the wall had been designed with access points to allow border officials to intercept migrants as they attempted to sneak into the UK. But an aid worker said the unlocked doors had rendered the whole thing “totally pointless”.
The worker said: “It cost a fortune and does nothing. They should have spent money rehousing people and giving them the support they need.”
The Home Office said: “Security for the wall is a responsibility for the French authorities.
“We continue to work with our French counterparts to maintain border security and keep legitimate passengers and trade moving.
“Last year Border Force disrupted over 56,000 illegal attempts to enter the UK. Officers use some of the most advanced detection technology around.”